The present invention relates to improved acrylic adhesive compositions.
The present invention further relates to acrylic adhesive compositions containing free radical initiators, having improved stability.
The present invention also relates to two component acrylic adhesive systems, i.e., a system in which the initiator and accelerator are kept separately prior to use.
All polymerizable adhesives have a limited shelf life. When stored at room temperature, the polymerizable adhesives ultimately undergo polymerization. However, premature gelation, a highly undesirable feature, can occur if the adhesive is not suitably stabilized. In some cases, the effect on components of an adhesive upon aging will result in a reduction of the rate at which the adhesive can cure when the accelerator is added.
In general, the tendency of an adhesive, or polymerizable monomer system to gel or polymerize, increases as the temperature increases. Conversely, lower temperature storage of the adhesive material results in an extension of the shelf life. In fact, in many segments of the adhesive market, refrigeration of adhesive products is often required. However, this technique to increase adhesive shelf life is both cumbersome and expensive. Moreover the adhesives must be rewarmed to working temperatures, usually room temperature, after refrigeration in order to attain a reasonable rate of cure or fixture time.
In the prior art, approaches are disclosed which utilize the addition of various substances in order to stabilize polymerizable adhesives, but many of these additives adversely affect the speed of cure of the adhesive, or they result in a deleterious lowering of the adhesive strength.
An ideal stabilization system is one which incorporates additives which have no adverse effect on cure speed or the adhesive physical properties, while imparting to the adhesive the desired shelf life. Much of the technology related to stabilization of acrylic adhesives has been developed for the "anaerobic" adhesives, where contamination of the composition by various metal ions has a negative effect on storage stability. In particular, transition metals having two or more valences are considered deleterious, and it is believed that such transition metals function as reductants by reacting with a free radical initiator, in particular, hydroperoxides, in a redox system.
Inhibitors previously utilized for free radical polymerizations include: quinones, hydroquinones, catechols, sterically-hindered phenols, phenothiazines, aminophenols, inorganic salts or oxides, quinone-oximes, nitroso-amines, etc. Of these, the most commonly used to stabilize commercial monomers are the quinones, hydroquinones and hydroquinone alkyl ethers. These inhibitors are effective barriers to polymerization of stored monomers. However, for the monomers containing peroxidic materials, such as typical free radical initiators, not only are the phenolic or amine inhibitors required, but also materials to remove or inactivate substances that may cause premature polymerization. In particular, transition metals or their ions are known to decompose hydroperoxides to produce radical products, which, in turn, initiate polymerization.
In the case of acrylic adhesives, or acrylic monomer compositions that cure and hold substrates together, the curing or hardening reaction is a polymerization of the monomers or oligomers. Premature formation of free radicals, by a redox reaction involving the peroxide or hydroperoxide, must be avoided if the adhesive is to have an adequate shelf storage life.
Thus, in addition to the typical free radical inhibitor, as described above, the incorporation of a second substance to inactivate contaminating transitional metal ions is often required for adequate storage stability of compositions containing acrylic monomers and/or oligomers, and a peroxidic polymerization initiator, such as a hydroperoxide.
U.S. Pat. No. 3,971,765 discloses that alpha amino-carboxylic monomers of the ethylenediamine tetraacetic acid (EDTA) type can be used to improve the stability of anaerobic adhesive formulations. Anaerobic adhesives are adhesives that are designed to cure or polymerize in the absence of oxygen.
As pointed out earlier, it is believed that trace metal ion contamination adversely affects the shelf life of the adhesive or at least renders the performance of the adhesive unreliable. The EDTA presumably chelates the metal ions, thereby rendering them ineffective in terms of reacting with the peroxidic initiator. The concentration levels taught in the '765 patent makes this technology undesirable. The '765 patent also points out the possibility of future contamination, i.e., if the chelator/sequestrant is insoluble in the adhesive mixture and separates from the mixture it cannot tie up newly introduced metal ions.
U.S. Pat. No. 3,991,261, assigned to Henkel and Cie, discloses the use of N-alkyl-c-aryl nitrones as stabilizers for anaerobic adhesives. These compounds are claimed to enhance adhesive stability while not interfering with normal anaerobic cure chemistry.
U.S. Pat. No. 3,962,372, assigned to DuPont, states that Hypalon.TM.(Dupont Chemical Company)-based two-part adhesives, e.g., see U.S. Pat. No. 3,890,407, are relatively unstable and set up or cure prematurely. Many adhesive stabilizers known to the art were found to adversely affect the rate of polymerization or cure time. An exception to this class is butylated hydroxy toluene (BHT). Quinone compounds were found to be ineffective stabilizers. In each case the preferred peroxide was cumene hydroperoxide (CHP).
U.S. Pat. No. 4,034,145, assigned to Henkel and Cie, discloses the use of a percarboxylic acid in order to improve the storage stability of the anaerobic adhesive without adversely affecting the curing properties.
U.S. Pat. No. 4,262,106, assigned to Loctite, discloses a process for treating anaerobic adhesive mixtures containing a sulfimide, such as saccharin, with an insoluble chelating agent, in order to improve the stability of anaerobic compositions. The technology in the '106 patent taught is both expensive, as large amounts of chelating agent are required, and cumbersome, as a separate step to remove the excess agent is required.
U.S. Pat. No. 4,038,475, assigned to Loctite, deals with the use of soluble chelators in conjunction with a quinone type inhibitor, such as naphthoquinone, in a formulated anaerobic composition. The soluble chelators claimed are derivatives of alpha and beta-amino caroboxylates, but chelators containing a --C.dbd.N-- linkage are excluded since they interfere with the speed of cure. The '475 patent also teaches that some quinone-type inhibitors adversely affect the speed of cure of adhesive compositions.
Another U.S. patent, i.e., U.S. Pat. No. 4,103,081, also assigned to Loctite, discloses the use of nitrobenzene and chlorinated nitrobenzenes as stabilizers for anaerobic compositions.